When you're reading the posts for a topic (or thread), you will see, at the very bottom of the page, a blue bar. The left end of the blue bar contains three links: Board index, Subscribe topic, and Bookmark topic. Click Bookmark topic, if you want to be able to find this topic easily later.

To find a bookmarked topic later: At the very top of most pages there is a blue bar with a link on the left called User Control Panel (right underneath the link to Board index). Click to go to the User Control Panel, and on the left there are a number of tabs. Front page is open by default. Click Manage bookmarks to see a list of all the topics you've bookmarked. They are listed in the order in which you bookmarked them, so scroll down to get to the latest.